This invention relates to a process for developing electrostatic latent images and more particularly to a process for developing electrostatic latent images with a two-component magnetic developer composed of a toner and a carrier.
One of the well-known dry processes for development of electrostatic latent images is a magnetic brush development which comprises triboelectrically charging toner particles to a polarity opposite to that of an electrostatic latent image, forming a magnetic brush of the developer by magnetic attraction, and brushing a latent image-carrying element with the magnetic brush to form a visible toner image on the element. A dry developer for the magnetic brush development is generally composed of two components, i.e., an insulating toner having an average particle diameter of about 10 to 20 microns, and a magnetic carrier such as, for example, iron, powder having an average particle diameter of about 100 to 200 microns.
In the magnetic brush development, the toner particles attracted to the surfaces of the carrier particles are consumed during development, whereas the carrier particles are not and replenished with toner particles for the repeated use. Thus, the consumption of the toner causes the developer to change in the composition, resulting in the deterioration of the image quality of the copies. In the practical applications, the developer is replenished with a fresh toner to keep the mixing ratio of toner to carrier constant. However, it is difficult to accurately control the mixing ratio of toner to carrier since the available range of the mixing ratio is too narrow to maintain the same within a proper range. Also, the developer must be periodically replaced with new one because repeated use of the carrier particles causes the toner to form a thin film on the surface of the carrier particle during prolonged use. The replacement of the developer is uneconomical and makes it troublesome to maintain the developing device to normal conditions.
To solve these problems, it has been proposed to use a ferrite as a material for carrier. The use of the ferrite carrier makes it possible to obtain electrophotographic copies with a high image density, but has problems awaiting a solution. The image quality can be improved by the use of ferrite carrier having a smaller particle size. The smaller the particle size of the carrier, the greater the magnetic attraction between the carrier particles, resulting in the aggregation of the carrier particles in a hard fin-like form. The hard fin-like arrangement of the carrier particles reduces its carrying properties and causes the formation of white lines in the solid black area of the copy. Also, the ferrites have a volume resistivity of about 10.sup.9 ohm-cm, so that the charges on the latent image-carrying element would leak away from the surface because of lowering of the toner content in the developer, resulting in the disappearance of the image and attraction of the carrier particles on the image area. If the ferrite carrier particles are attracted to the latent image-carrying element, the surface of the element would be damaged by the hard ferrite particles when removing residual toner particles on the element. In addition, the line images cannot be reproduced sharply. The volume resistivity of the ferrite carrier may be improved by coating the carrier particles with an insulating resin, but the aggregation of the carriers cannot be prevented by coating. Since the coated ferrite carrier particles have a tendency to be heavily charged, the carrier particles are attracted to the non-image area of the element by the triboelectrical attraction, resulting in the damage of the element.